The art and science of drying foodstuffs is vital to our society. Freshly harvested foods require dehydration if they are to be stored without spoilage.
Drying equipment is a substantial capital investment. However, most dryers are tailored for particular needs and operate only on a limited seasonal basis. The rest of the year they often sit idle. So far as is known, an all-purpose dryer has never been designed.
Direct-fired rotary dryers have high volume capacity, but are cumbersome and require operating temperatures which may scorch the product.
Flash dryers offer rapid drying, but do not handle liquids or slurries nor remove occluded moisture from inside the food.
Spray dryers are useful for only a limited range of materials.
Fluid-bed dryers are a step forward in achieving continuous drying. Fluidization offers uniform temperatures and high heat- and mass-transfer rates. However, fluid-bed dryers do not easily accommodate sticky feeds; tend to overdry delicate product and often "short circuit" particles through the drying path. Moreover, the fluidizing medium is much hotter than the bed itself and can scorch or burn the product if fluidization is interrupted.
In the typical fluid-bed drying, a bed of material rests on a perforated grid. Hot pressurized gas flows upwardly through the grid and fluidizes the bed. Yet it is difficult to keep a bed fluidized. A large number of particles tend to clump together rather than float individually in streams of air. The fluidized bed tends to become nonuniform. The gas eventually geysers or erupts through some weak spot in the bed and fluidization collapses. In addition, sticky particles sometimes "glue" together and resist fluidization. Moreover, sticky fluids can clog the grid and block the gas flow.
Attempts were made to solve these problems by uniformly exposing the material to the hot gas. Care was taken to not disturb the bed to avoid generating weak spots which could disrupt fluidization.
One proposed fluid-bed dryer is shown in U.S. Pat. No. 3,592,395, filed Sept. 16, 1968, to Lockwood et al. Hot gas and fresh slurry enter the dryer at its center and flow toward an outer wall. The gas is much hotter than the slurry and initially contacts a relatively small slurry volume. This tends to scorch or burn the product. Moreover, during startup and shutdown, material sometimes tumbles down the centrally-located hot gas inlet and causes fires.
There are problems in handling an assortment of particles having differing masses and volumes. Light particles may need only a short exposure, while heavier particles require longer processing which would overdry the lighter product. There is need for a dryer which can dehydrate light and heavy particles together yet provide a uniformly dried product. There is also need for a dryer which is versatile enough to handle sticky material.